The Role of Propagule Pressure, Genetic Diversity and Microsite Availability for <em>Senecio vernalis</em> Invasion

<div><p>Genetic diversity is supposed to support the colonization success of expanding species, in particular in situations where microsite availability is constrained. Addressing the role of genetic diversity in plant invasion experimentally requires its manipulation independent of propagule pressure. To assess the relative importance of these components for the invasion of <i>Senecio vernalis</i>, we created propagule mixtures of four levels of genotype diversity by combining seeds across remote populations, across proximate populations, within single populations and within seed families. In a first container experiment with constant <i>Festuca rupicola</i> density as matrix, genotype diversity was crossed with three levels of seed density. In a second experiment, we tested for effects of establishment limitation and genotype diversity by manipulating <i>Festuca</i> densities. Increasing genetic diversity had no effects on abundance and biomass of <i>S. vernalis</i> but positively affected the proportion of large individuals to small individuals. Mixtures composed from proximate populations had a significantly higher proportion of large individuals than mixtures composed from within seed families only. High propagule pressure increased emergence and establishment of <i>S. vernalis</i> but had no effect on individual growth performance. Establishment was favoured in containers with <i>Festuca</i>, but performance of surviving seedlings was higher in open soil treatments. For <i>S. vernalis</i> invasion, we found a shift in driving factors from density dependence to effects of genetic diversity across life stages. While initial abundance was mostly linked to the amount of seed input, genetic diversity, in contrast, affected later stages of colonization probably via sampling effects and seemed to contribute to filtering the genotypes that finally grew up. In consequence, when disentangling the mechanistic relationships of genetic diversity, seed density and microsite limitation in colonization of invasive plants, a clear differentiation between initial emergence and subsequent survival to juvenile and adult stages is required.</p> </div

Experiment 2: Microsite availability×genetic diversity.

<p>Box and Whisker plots of A) Initial abundance (week 1), B) Establishment (week 15), C) Total biomass (given as Log₁₀ for reasons of clarity, week 15) and D) Log size ratio (week 15) of <i>Senecio vernalis</i> separately by genetic diversity levels applied: SF = <i>seed family</i> (n = 18 each), WP = <i>within population</i> (n = 12 each), PP = <i>proximate populations</i> (n = 6 each), RP = <i>remote populations</i> (n = 6 each). 15, 10, 5, 0 = numbers of <i>Festuca</i> individuals as density levels applied. Different letters indicate significant differences between levels of main factors according to the Scheffé test. For further statistical details see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057029#pone-0057029-t002" target="_blank">Table 2</a>.</p

Experiment 1: Propagule pressure×genetic diversity.

<p>GLM for response variables of <i>Senecio vernalis</i>. Initial abundance reflects the number of <i>Senecio</i> individuals after 1 week. Establishment refers to the number of <i>Senecio</i> individuals after 15 weeks. Abundance and total biomass were log₁₀(x+1)-transformed prior to analysis. Log size ratio gives the proportion of number of large individuals compared to those of small individuals as logarithm to the base 10. N = 126. The results of the Scheffé post hoc tests indicate the direction of significant differences between categories of each factor. 90, 60, 30 indicates seed density levels applied. Seed mixtures: RP = <i>remote populations</i>, PP = <i>proximate populations</i>, WP = <i>within population</i>, SF = <i>seed family</i>. The tests of fixed effects are based on type III SS, p values and degrees of freedom of numerator (df Num) and denominator (df Den) are shown. Bold numbers indicate significant effects (p<0.05).</p

Experiment 1: Propagule pressure×genetic diversity.

<p>Box and Whisker plots of A) Initial abundance (week 1), B) Establishment (week 15), C) Total biomass (given as Log₁₀ for reasons of clarity, week 15) and D) Log size ratio (week 15) of <i>Senecio vernalis</i> separately by genetic diversity levels applied: SF = <i>seed family</i> (n = 18 each), WP = <i>within population</i> (n = 12 each), PP = <i>proximate populations</i> (n = 6 each), RP = <i>remote populations</i> (n = 6 each). 30, 60, 90 = seed density levels applied. Different letters indicate significant differences between levels of main factors according to the Scheffé test. For further statistical details see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057029#pone-0057029-t001" target="_blank">Table 1</a>.</p

Experiment 1: Temporal development of <i>Senecio vernalis</i> abundance.

<p>Data are given separately for seed density levels applied. Open circles depict 90 seeds per container, filled grey circles depict 60 seeds per container, black dots show 30 seeds per container (n = 42 per seed level).</p

Experiment 2: Microsite availability×genetic diversity.

<p>GLM for response variables of <i>Senecio vernalis</i>. Initial abundance reflects the number of <i>Senecio</i> individuals after 1 week. Establishment refers to the number of <i>Senecio</i> individuals after 12 weeks. Abundance and total biomass were log₁₀(x+1)-transformed prior to analysis. Log size ratio gives the proportion of the number of large individuals compared to those of small individuals as logarithm to the base 10. N = 168. The results of the Scheffé post hoc tests indicate the direction of significant differences between categories of each factor. 15, 10, 5, 0 indicates numbers of <i>Festuca</i> individuals as density levels applied. Seed mixtures: RP = <i>remote populations</i>, PP = <i>proximate populations</i>, WP = <i>within population</i>, SF = <i>seed family</i>. The tests of fixed effects are based on type III SS, p values and degrees of freedom of numerator (df Num) and denominator (df Den) are shown. Bold numbers indicate significant effects (p<0.05).</p

A novel comparative research platform designed to determine the functional significance of tree species diversity in European forests

One of the current advances in functional biodiversity research is the move away from short-lived test systems towards the exploration of diversity-ecosystem functioning relationships in structurally more complex ecosystems. In forests, assumptions about the functional significance of tree species diversity have only recently produced a new generation of research on ecosystem processes and services. Novel experimental designs have now replaced traditional forestry trials, but these comparatively young experimental plots suffer from specific difficulties that are mainly related to the tree size and longevity. Tree species diversity experiments therefore need to be complemented with observational studies in existing forests. Here we present the design and implementation of a new network of forest plots along tree species diversity gradients in six major European forest types: the FunDivEUROPE Exploratory Platform. Based on a review of the deficiencies of existing observational approaches and of unresolved research questions and hypotheses, we discuss the fundamental criteria that shaped the design of our platform. Key features include the extent of the species diversity gradient with mixtures up to five species, strict avoidance of a dilution gradient, special attention to community evenness and minimal covariation with other environmental factors. The new European research platform permits the most comprehensive assessment of tree species diversity effects on forest ecosystem functioning to date since it offers a common set of research plots to groups of researchers from very different disciplines and uses the same methodological approach in contrasting forest types along an extensive environmental gradient